A facile and efficient pinacol–pinacolone rearrangement of vicinal diols using ZnCl2 supported on silica as a recyclable catalyst

Abstract

An efficient pinacol rearrangement of variety of primary, secondary and tertiary, symmetrical as well as asymmetrical diols has been investigated using ZnCl2/SiO2 (silzic) as solid acid catalyst. The catalyst is prepared by co-grinding silica gel with anhydrous ZnCl2 followed by thermal activation. The nature of interaction has been investigated using powder XRD, TG-DTA and DSC analyses, FT-IR spectroscopy and by testing the catalytic properties in pinacol–pinacolone rearrangement. FT-IR spectra of catalyst using pyridine as probe molecule shows presence of predominant Lewis acid sites. The results of FT-IR, TG-DTA, DSC analyses and XRD indicated that dispersed ZnCl2 co-ordinates with surface hydroxyl groups leading to formation of a new zinc ion species of –O–Zn–Cl as Lewis acid sites. Various highly substituted aldehydes and ketones were obtained in high isolated yield with excellent selectivity. The catalyst was found to be highly efficient and reusable without any loss of activity. The fact that ZnCl2 did not leach in presence of 1,2-dichloroethane as solvent makes silzic an alternative heterogeneous acid catalyst for pinacol rearrangement to the conventional homogeneous acids. The other interesting facet of this communication is highly efficient synthesis of cyclic spiroketones using silzic as reusable solid acid catalyst hitherto unknown.